The purpose of a catalytic converter for an internal combustion engine, or a gas turbine, is to convert pollutant materials in the exhaust, e.g., carbon monoxide, unburned hydrocarbons, nitrogen oxides, etc., to carbon dioxide, nitrogen and water prior to discharge into the atmosphere. Conversion to such relatively harmless by-products is not efficient initially when the exhaust gases are relatively cold, e.g., at cold engine start. To be effective at a high conversion rate, the catalyst and surface of the converter which the gases contact must be at or above a minimum temperature, e.g., 390.degree. F. for carbon monoxide, 570.degree. F. for volatile organic compounds, and about 900.degree. F. for methane or natural gas. Otherwise, conversion to harmless by-products is poor and cold start pollution is high.
To achieve initial heating of the catalyst at engine start-up, there is conveniently provided an electrically heatable catalytic converter unit, preferably one formed of a thin metal honeycomb monolith. Recent developments have demonstrated the advantage of providing a "cascade" of converters, i.e., a low thermal inertia electrically heatable converter (EHC), followed by a medium thermal inertia converter, or light-off converter followed by a large thermal inertia main converter, all in the same conduit or gas flow line. Heat generated from an oxidation reaction initiated in the EHC then heats the intermediate converter which in turn heats the large converter. The EHC preheats the exhaust gas to its "light-off" temperature for entry into the "light-off" converter where in the presence of catalyst pollutant material is converted. Some conversion occurs in the EHC, and most of the conversion occurs in the final catalytic converter section which is not normally electrically heated. The present invention is primarily concerned with the "light-off" converter which follows an electrically heatable converter section.
A common problem with thin metal honeycomb monoliths has been their inability to survive severe automotive industry durability tests which are known as the Hot Shake Test and the Hot Cycling Test.
The Hot Shake Test involves oscillating (100 to 200 Hertz and 28 to 60 G inertial loading) the test device in a vertical attitude at high temperature (between 800.degree. and 950.degree. C.; 1472.degree. to 1742.degree. F. respectively) with exhaust gas from a running internal combustion engine simultaneously passing through the device. If the test device telescopes or displays separation or folding over of the leading or,upstream edges of the thin metal foil leaves up to a predetermined time, e.g., 5 to 200 hours, the test device is said to fail the test. Usually a device that lasts 5 hours will last 200 hours. Five hours is equivalent to 1.8 million cycles at 100 Hertz.
The Hot Cycling Test is conducted with exhaust gas flowing at 800.degree. to 950.degree. C. (1472.degree. to 1742.degree. F.) and cycles to 120.degree. to 150.degree. C. once every 15 to 20 minutes for 300 hours. Telescoping or separation of the leading edges of the thin metal foil strips is considered a failure.
The Hot Shake Test and the Hot Cycling Test are hereinafter called "Hot Tests," and have proved very difficult to survive. Many efforts to provide a successful device have been either too costly or ineffective for a variety of reasons.
The reinforced structures of the present invention will survive these Hot Tests.
Reference may be had to U.S. Pat. No. 5,102,743 dated Apr. 7, 1992 to Maus et al. This patent discloses a monolith made of thin metal strips, alternating corrugated and flat sheet metal layers. The reference discloses that at least one of the sheet metal layers has a greater thickness over at least part of at least one of the dimensions (length and width) than the others of the layers. The at least one sheet metal layer having a greater thickness is formed of thicker sheet metal than others of the layers or it is formed of a plurality of identically structured metal sheets resting closely against one another. The present invention depends upon the use of sheet metal layers of different alloys having different strengths than others of the sheet metal layers to achieve a durable converter body.
In the following description, reference will be made to "ferritic" stainless steel. A suitable ferritic stainless steel alloy is described in U.S. Pat. No. 4,414,023 dated Nov. 8, 1983 to Aggen. A specific ferritic stainless steel useful herein contains 20% chromium, 5% aluminum, and from 0.002% to 0.05% of at least one rare earth metal selected from cerium, lanthanum, neodymium, yttrium, and praseodymium, or a mixture or two or more of such metals, balance iron and trace steel making impurities. This alloy has a yield strength at 900.degree. C. of 2,000 psi, an ultimate tensile strength of 5,300 psi, and 1% creep strength at 1000 hours of 330 psi. Another metal alloy especially useful herein is identified as Haynes 214 alloy which is commercially available. This alloy and other nickeliferous alloys are described in U.S. Pat. No. 4,691,931 dated Jun. 9, 1987 to Herchenroeder et al. A specific example contains 75% nickel, 16% chromium, 4.5% aluminum, 3% iron, optionally trace amounts of one or more rare earth metals, except yttrium, 0.05% carbon, and trace amounts of steel making impurities. This alloy has a yield strength of 46,000 psi, an ultimate tensile strength of 52,000 psi and a 1% creep strength at 1000 hours of 2500 psi, all properties measured at 900.degree. C. Haynes 230 Alloy, also useful herein, has a composition containing 22% chromium, 14% tungsten, 2% molybdenum, 0.10% carbon, 5% max cobalt, 3% max iron, and a trace amount of lanthanum, and balance nickel. Haynes 230 alloy has a yield strength of 32,100 psi, an ultimate tensile strength of 49,000 psi and a 1% creep strength at 1000 hours of 3700 psi, all properties measured at 900.degree. C. Ferritic stainless steel (commercially available from Allegheny Ludlum Steel Co. under the Trademark "Alfa IV") and the Haynes alloys are examples of high temperature resistive, oxidation resistant (or corrosion resistant) metals that are suitable for use in the converters hereof. Suitable metals must be able to withstand temperatures of 900.degree. C. to 1100.degree. C. over prolonged periods.
It has now been found that a multicellular converter body formed of alternating corrugated and flat thin metal layers and having some of the thin metal strips replaced with a different and stronger alloy will survive the Hot Tests.